The most commonly practiced technology for fabricating multi-layer substrates uses co-fired tape cast ceramics. The co-fired ceramic structure is a monolithic ceramic substrate after it has been completely fired. However, the manufacture of multi-layer substrates using cast ceramic, or xe2x80x9cgreen tapexe2x80x9d, introduces its own problems. This technology possesses a number of disadvantages due to potential variation in the alignment of conductive patterns, vias and cavities which limit interconnect density. These problems are created by the differential shrinkage within and between the individual layers of the ceramic material from which the multi-layer substrate is formed. Also, the surface roughness of the tape cast ceramics limit electrical performance. Further, since tape cast or green sheet ceramics can contain between 8% and 40% binders, the purity levels of the processed ceramics are not tightly controlled, leading to a compromise in electrical performance. At higher frequency applications, electrical response can become quite sensitive to material variations, resulting in the limitation of the electrical performance of co-fired ceramics to lower frequencies within the millimeter wave and microwave ranges.
A process has been developed for fabricating millimeter wave and microwave packages and interconnect structures in which a multi-layer structure of fully-fired (or hardened) ceramics with conductive patterns is formed by attaching separate substrate layers together with seal glass which has a coefficient of thermal expansion (CTE) which is matched as closely as possible to the CTE of the substrates. The pre-fired ceramics are fully hardened prior to processing so that no shrinkage occurs as binders are burned off, as would occur in green sheet processing. In contrast, as green tape ceramics are fired (hardened), shrinkage occurs as the material is sintered and as binders are burned off. This is an undesirable phenomenon with respect to routing RF circuitry since the metallization pattern will also shrink and shift, affecting the electrical response of the circuit. With the fully-fired ceramics, no compensation is required to allow for shrinkage of conductive features, so that tighter control of dimensions is available, and higher density features can be incorporated. The fully-fired ceramic material, including, but not limited to, alumina, aluminum nitride, berrylia, and quartz, is selected to conform with the intended operation parameters of the package or component to be fabricated.
Special considerations arise when fabricating filters for microwave and millimeter wave applications for satellite and mobile communication systems. While small size and mass are desirable, stability of electrical response is a significant concern for narrowband or highly specialized performance requirements. Stability of the center frequency of a bandpass filter can vary with temperature. Such drift can be particularly detrimental for switch filter networks which are used in systems that provide a high degree of channel flexibility in order to precisely divide the spectrum. In conventional approaches, the frequency drift problem due to temperature fluctuation is overcome by using Invar(copyright) (InFe) waveguides, which are expensive. Although the use of waveguides is unavoidable in high power satellite applications, the proposed filter can be useful in low power applications such as receiver front ends. Furthermore, in the lower microwave region, filters use lumped elements which are also expensive and not suitable for low cost mass production. Lumped element filters are very lossy when compared to distributed resonator filters.
In conventional fabrication technology, filters are manufactured using high dielectric constant ceramic-based coaxial resonators. The designs are based on empirical techniques and each filter is individually fabricated and must be tuned after production. The advantages of using a stripline approach are as follows:
1) Fully-fired ceramic technology applied to a stripline approach yields filters which should not require any post-production tuning;
2) Because a stripline is a printed transmission line, a large number of filters can be simultaneously printed on a single ceramic board. Consequently, a great reduction in production cost could be achieved;
3) Fully-fired ceramic technology applied to stripline processing and grounding techniques will give rise to a completely shielded and robust ceramic block filter capable of withstanding high levels of vibration and g-forces.
It is an advantage of the invention to provide a process for fabricating a stripline filter for microwave and millimeter wave applications which utilizes hardened, high dielectric constant ceramic substrates combined with stripline technology to create a small, lightweight package.
It is another advantage of the present invention to provide a process for mass producing a stripline filter with tight tolerances and repeatable performance.
It is a further advantage of the present invention to provide a process for fabricating a stripline filter which eliminates the need for compensation for dimensional instability of the ceramic substrate.
In an exemplary embodiment, the process for fabricating a stripline filter uses two pre-fired ceramic substrates. The substrates may be lapped for flatness and parallelism between the top and the bottom, and to modify the surface texture to provide better electrical performance at high frequency where such performance is required. A conductive paste is applied to the top of each substrate using thick film techniques as are known in the industry. Multiple screening steps are required to obtain the desired conductor thickness. On the top of the lower layer, the conductive paste is patterned with the filter structure using photolithographic and chemical etch techniques which are known in thick and thin film processing. The conductive paste is also screen printed onto the bottom side of the lower layer and the sides of the lower layer are painted with the same conductive paste. A seal glass is screen printed on top of the patterned conductor and is dried and glazed to burn off organic binder and to cause the material to bond together. On the upper layer, the conductive paste is unpatterned. Seal glass is printed onto the bottom of the upper layer, dried and glazed. The upper and lower layers are clamped together after aligning their edges and are fired to seal the two layers together. Groundplane connection between the upper and lower layers is provided by painting the sides of the assembly with conductive paste. The entire assembly is again fired to burn off the binder and to harden the conductor. If desired, the lower layer substrate can be laser machined to provide launch areas for wire bond attachment. This machining will be done before lapping.